1. Field of the Invention
The present invention relates in general to microwave oven having an absolute humidity sensor, and more particularly to an electronic range having a fan motor rotation control function which controls the number of rotations of the fan motor to allow the absolute humidity sensor to be at the optimum condition to sense a cooking state of a dish.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a construction of a conventional microwave oven having an absolute humidity sensor. As shown in the drawing, the conventional electronic range comprises a body 1, a magnetron 2 mounted on a right upper side inside the body 1, a high voltage transformer 3 disposed on a right lower side inside the body 1, a fan motor 5 disposed on a central portion of the rear side of the body 1, a fan 4 mounted to the fan motor 5, an air blowing duct 6 disposed toward a front upper side of the body 1, a door 18 disposed on the front side of the body 1, and an air blowing hole 19 disposed on a front right upper side of the body 1.
FIG. 2 is a schematic circuit diagram of the conventional microwave oven illustrated in FIG. 1. As shown in the drawing, one side of an AC power source is connected to a primary winding of a low voltage transformer 10 and to one terminal of a primary security switch 11 via a fuse 7, a magnetron thermostat 8 and an oven thermostat 9, and the other side thereof is connected to the primary winding of the low voltage transformer 10, to one terminal of a monitor switch driving relay 12 and to the fan motor 5 via a lamp 13 and a turntable motor 14. The one terminal of the primary security switch 11 connected to the oven thermostat 9 is also connected to the lamp 13 via a lamp driving relay 15 and to the turntable motor 14 and the fan motor 5 via a fan motor and turntable motor driving relay 16. The turntable motor 14 and the fan motor 5 are connected in parallel to each other. Also, between the other terminal of the primary security switch 11 and the other terminal of the monitor switch driving relay 12 are connected in parallel a monitor switch 17 and the high voltage transformer 3.
The conventional microwave oven is also provided with a control circuit 21 to which a door switch 20 is connected.
FIG. 3 is a detailed block diagram of the control circuit 21 illustrated in FIG. 2. As shown in this drawing, the control circuit 21 includes a microprocessor 22, a zero balance circuit 23, an absolute humidity sensor 24, a humidity detecting circuit 25, and a relay driving circuit 26.
Also, the conventional microwave oven is provided with a keyboard 27 mounted on an external control panel (not shown) thereof. This keyboard 27 is connected to the microprocessor 22.
The operation of the conventional microwave oven with the above-mentioned construction will now be described.
First, upon the applying of AC power to the low voltage transformer 10, the AC power is also applied to the control circuit 21. Under the condition that the door 18 of the microwave oven is closed, the microprocessor 22 outputs a plurality of control signals to the relay driving circuit 26 in response to a plurality of command signals received from the keyboard 27 mounted on the control panel. Upon receiving the control signals from the microprocessor 22, the relay driving circuit 26 outputs such signals as to turn on the lamp driving relay 15 and the fan motor and turntable motor driving relay 16. As the lamp driving relay 15 is turned on by the output signal from the relay driving circuit 26, the lamp 13 is turned on, thereby allowing a user to see a cooking state of a dish, or food or drink, inside the body 1 of the microwave oven. On the other hand, as the fan motor and turntable motor driving relay 16 is turned on by the output signal from the relay driving circuit 26, the turntable motor 14 and the fan motor 5 are applied with AC power, thereby allowing the fan 4 mounted to the fan motor 5 to be actuated to generate air. This air is blown through the air blowing hole 19 disposed on the front right upper side of the body 1, via the high voltage transformer 3, the magnetron 2 and the air blowing duct 6. At this time, under the condition that the monitor switch driving relay 12 is being turned off, only relays 15 and 16 are turned on, in order to blow moist air inside the body 1 of the microwave oven through the air blowing hole 19 for about 18 seconds. The reason why this operation must be performed will now be mentioned.
When another dish is put into the body 1 of the microwave oven after the cooking of a dish has been finished and the next cooking of another dish is to be tried, a large amount of water vapor generated from the first dish still remains inside the body 1. The absolute humidity sensor 24 when the next cooking is being carried out senses the relatively large amount of the water vapor inside the body 1 and outputs a signal according to the sensed state to the microprocessor 22 via the humidity detecting circuit 25 in the control circuit 21. For this reason, the microprocessor 22 malfunctions.
In order to solve the problem as mentioned above, the conventional microwave oven further comprises functions as follows:
Namely, under the condition that the monitor switch driving relay 12 is being turned off by the microprocessor 22 for about 18 seconds, the zero balance circuit 23 in the control circuit 21 zero-balances the absolute humidity sensor 24, thereby allowing the operation of the absolute humidity sensor 24 to normally begin with a constant reference voltage, or a zero balance point of time, regardless of the amount of the water vapor inside the body 1 of the microwave oven.
FIG. 4 is a waveform diagram of outputs from the absolute humidity sensor 24 illustrated in FIG. 3. As shown in the drawing, although the normal cooking is carried out after the zero-balancing of the absolute humidity sensor 24, water vapor is not generated inside the body 1 of the electronic range during a constant time period. That is, although the monitor switch driving relay 12 is turned on and the high voltage transformer 3 is thus applied with AC power thereby to actuate the magnetron 2, the dish inside the body 1 of the microwave oven is not at once heated, causing the generation of the water vapor therefrom. Because the fan motor and turntable motor driving relay 16 are always being turned on regardless of the menu of the dish, the turntable motor 14 and the fan motor 5 operate at the same load condition in accordance with AC power. The output voltage from the absolute humidity sensor 24 is decreased during a constant time period in accordance with the menu to be cooked, since amount of external water vapor is relatively less than that of water vapor inside the body 1 of the microwave oven during this constant time period. Namely, the output voltage from the absolute humidity sensor 24 is decreased until the dish is heated and the water vapor therefrom is thus generated. As the dish is being heated gradually, the increase in amount of the water vapor generated from the dish occurs step by step. As a result, the output voltage from the absolute humidity sensor 24 is increased.
The microprocessor 22 in the control circuit 21 perceives the output voltage from the absolute humidity sensor 24 and detects the minimum point (point of time t1 in FIG. 4) in accordance with the perceived state, during a constant time period after the zero-balancing of the absolute humidity sensor 24 (point of time t0 in FIG. 4). Then, the microprocessor 22 perceives the state (point of time t2 in FIG. 4) when the output voltage from the absolute humidity sensor 24 is increased and reaches a preselected value h in accordance with the menu of the dish.
As hereinbefore described, because the fan motor 5 is connected in parallel across the AC power source and is driven by the fan motor and turntable motor driving relay 16, the number of rotations thereof is varied in accordance with the voltage variation of the AC power source.
That is, when the larger voltage is applied from the AC power source, the fan motor 5 is rotated relatively faster, thereby causing the sensitivity of the absolute humidity sensor 24 to become worse relatively. For this reason, the variation width h at the point of time sensed by the absolute humidity sensor 24 is larger at the minimum point (point of time t1 in FIG. 4) after the zero-balancing of the absolute humidity sensor 24, thereby causing the absolute humidity sensor 24 to be liable to be influenced by an external noise. Also, because of non-linearity of the waveform of outputs from the absolute humidity sensor 24 as shown in FIG. 4, it is difficult for the microprocessor 22 is difficult to perceive the outputs from the absolute humidity sensor 24. In addition, because the number of rotations of the fan motor 5 is varied in accordance with the voltage variation of the AC power source, the speed of the wind thereof cannot be adjusted adequately according to the menu of the dish.